This invention relates to a device for measuring an oxygen concentration within an exhaust gas from an internal combustion engine etc., to sense the air/fuel ratio and in particular to an improved engine air/fuel ratio sensing device of an oxygen pump type constructed using an ion conductive solid electrolyte.
It is hitherto well known in the art to control e.g. the engine of an automobile to run at a stoichiometric (theoretical) air/fuel ratio, by sensing its combustion state in relation to the stoichiometric air/fuel ratio according to the variation of an electromotive force produced by the difference of the oxygen partial pressure between the exhaust gas and the atmosphere, by means of an oxygen sensor constructed with an ion conducting solid electrolyte such as stabilized zirconia. It is to be noted here that air/fuel ratio (A/F) is given by the weight ratio of air to fuel and that the principle of oxygen sensing is described in "Applied Physics Lett. 38(5), Mar. 1, 1981".
When the air/fuel ratio is the stoichiometric air/fuel ratio of 14.7, the above type oxygen sensor can provide a large output variation while outside the stoichiometric air/fuel ratio it provides a very low output variation. Therefore, when the engine is operated at an air/fuel ratio outside the stoichiometric air/fuel ratio, the output of such an oxygen sensor can not be utilized.
There has already been proposed an air/fuel ratio sensor of an oxygen pump type which eliminates such a disadvantage and enables the engine to be operated at any air/fuel ratio.
FIG. 1 shows an arrangement of an air/fuel ratio sensing device of an oxygen pump type, and FIG. 2 shows a cross sectional view of the sensor in FIG. 1 taken along line II--II, which is disclosed in a related application Ser. No. 606,926 filed May 4, 1984.
In FIG. 1, within an exhaust pipe 1 of an engine (not shown) an air/fuel ratio sensor, generally designated by a reference numeral 2, is disposed. This sensor 2 is formed of a solid electrolyte oxygen pump cell 3, a solid electrolyte oxygen sensor cell 4, and a supporting base 5. The solid electrolyte oxygen pump cell 3 includes an ion conducting solid electrolyte (stabilized zirconia) 6 in the form of a plate having platinum electrodes 7 and 8 disposed on the respective sides thereof. The solid electrolyte oxygen sensor cell 4, likewise the pump cell 3, includes an ion conductive solid electrolyte 9 in the form of a plate having platinum electrodes 10 and 11 disposed on the respective sides thereof. The supporting base 5 supports the oxygen pump cell 3 and the oxygen sensor cell 4 so that they are oppositely disposed having a minute gap "d" of about 0.1 mm therebetween.
An electronic control unit 12 is electrically coupled to the pump cell 3 and the sensor cell 4. More specifically, the electrode 10 is connected through a resistor R1 to the inverting input of an operational amplifier A the non-inverting input of which is grounded through a DC reference voltage source V. This DC reference voltage serves to control the output voltage of the sensor cell 4 to assume said voltage V according to the oxygen partial pressure difference between those within the gap and outside the gap. The electrode 7 is connected through a resistor R0 to the emitter of a transistor Tr whose collector is grounded through a DC power source B and whose base is connected to the output of the operational amplifier A and the inverting input of the operational amplifier A through a capacitor C. The electrodes 8 and 11 are grounded.
In operation, when the oxygen partial pressure within the gap portion between the cells 3 and 4 is the same as the oxygen partial pressure outside the gap portion, the sensor cell 4 generates no electromotive force. Therefore, the inverting input of the operational amplifier A receives no voltage and so the operational amplifier A provides as an output a maximum voltage corresponding to the reference voltage V to the base of the transistor Tr. Therefore, the transistor Tr is made conductive to cause a pump current Ip to flow across the electrodes 7 and 8 of the pump cell 3 from the voltage source B. Then the pump cell 3 pumps oxygen within the gap portion into the exhaust pipe 1. As a result, the sensor cell 4 develops an electromotive force thereacross according to the oxygen partial pressure difference on both sides of the cell 4.
Therefore, the oxygen sensor cell 4 applies an electromotive force "e" generated across the electrodes 10 and 11 to the inverting input of the operational amplifier A through the resistor R1. The operational amplifier A provides an output now proportional to the difference between the electromotive force "e" and the reference DC voltage V applied to the non-inverting input. The output of the operational amplifier A drives the transistor Tr to control the pump current Ip.
Thus, the electromotive force "e" approaches the reference voltage V. Accordingly, the control unit 12 reaches an equilibrium state and serves to provide a pump current Ip necessary for keeping the electromotive force "e" at the predetermined reference voltage V. The resistor R0 serves to provide an output corresponding to the pump current Ip supplied from the DC power source B as a pump current supply means. The pump current Ip corresponds to an air/fuel ratio value. This pump current Ip is converted into the voltage by the resistor R0 and is sent to a fuel control unit (not shown) so that the fuel control unit is controlled at a desired air/fuel ratio. The resistance of the resistor R0 is selected so as to prevent the pump current Ip from flowing excessively from the DC power source B. The capacitor C forms an integrator associated with the operational amplifier A and serves to make the electromotive force "e" precisely coincident with the reference voltage V.
One example of the static characteristics of a conventional air/fuel ratio sensing device of an oxygen pump type thus constructed in the form of a negative feedback control is shown in FIG. 3. The different characteristic curves a and b are obtained by changing the reference voltage V in FIG. 1, as disclosed in related application Ser. No. 606,910 filed May 4, 1984. The characteristic curve a is preferable when the air/fuel ratio (A/F) is controlled in a so-called "rich" region where the A/F ratio is below the stoichiometric A/F ratio 14.7 and in a so-called "lean" region where the A/F ratio is above the stoichiometric A/F ratio 14.7 while the characteristic curve b is preferable when the A/F ratio is controlled at the stoichiometric A/F ratio 14.7.
The air/fuel ratio sensor 2 illustrated in FIG. 1 has basically excellent characteristics because in either the rich region or the lean region the A/F ratio is linearly interrelated with the pump current Ip to thereby enable the engine to be operated at any A/F ratio.
However, even with the sensor 2 in FIG. 1, the oxygen pump cell 3 and the oxygen sensor cell 4 have at least an electrical first order lag, respectively. There is also a considerable lag time required for measured gas within the exhaust pipe 1 to disperse into the minute gap "d".
Furthermore, since the air/fuel ratio sensing device shown in FIG. 1 always requires a negative feedback control through an integrator in order to properly make the electromotive force "e" of the oxygen sensor cell 4 coincident with the reference voltage V, it has the following disadvantages in the dynamic characteristic range. These disadvantages are also present with, the device shown in Hetrick, U.S. Pat. No. 4,272,329 which includes an integrator formed by an operational amplifier and capacitor.
First of all, the response lag as an air/fuel ratio sensor is always high due to the integration by the integrator so that its performance as required for the A/F ratio control of an engine is not totally satisfactory.
Secondly, there is at least a phase lag of 180 degrees due to lag elements such as the oxygen pump cell 3 (90 degrees) and the sensor cell 4 (90 degrees) and in addition the phase lag of 90 degrees due to the integrator formed of the operational amplifier A and the capacitor C, with the result that the sensor 2 has a danger of oscillation. Some experiments have revealed that the combination of various parameters such as the flow temperature of measured gas or an A/F ratio often causes the sensor to oscillate.